Line of Sight Pull

The "line of sight pull" is a technique that can be used to pull mobs of all types.

Traditional "pulling" involves attacking a mob from range so they run closer to your party away from their support. This is usually done so the party can enter battle without the possibility of accidentally pulling another group or patrol. It can also be used to break up groups so your party can progress through smaller groups at a time.

The problem with this strategy is that it doesn't work against mobs that have a ranged attack. They will just stand there and shoot you, forcing you to advance and enter the danger zone. Depending on the mob, letting them stand there and attack you can be fatal.

The line of sight pull works by breaking the mob's "line of sight" after you range attack them. It's pretty simple, here is the process:

1. Perform your normal pull attack. This varies for each character but is usually a ranged attack. Another option is a Face Pull.
2. Retreat around a corner or behind a LOS blocking* structure. If your target is a spell caster you will notice that their spell will be interrupted. Watch the spell cast progress bar on their portrait.
3. The mob will follow you around the corner to your new safe zone where you can safely enter combat.

WOWWiki has some information about Line of Sight.

* Note: Not all structures in WoW break line of sight. The game uses two world geometries, one for the visual display and a much simpler one for LOS calculations. This is to speed up these calculations, but has the side effect that some walls/trees/structures don't break LOS. You need to be prepared for the worst.